US5009074A - Low refrigerant charge protection method for a variable displacement compressor - Google Patents

Images

Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
  • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
  • F25B49/00—Arrangement or mounting of control or safety devices
  • F25B49/005—Arrangement or mounting of control or safety devices of safety devices
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
  • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
  • F25B2500/00—Problems to be solved
  • F25B2500/22—Preventing, detecting or repairing leaks of refrigeration fluids
  • F25B2500/222—Detecting refrigerant leaks

Definitions

• This invention pertains to the control of a variable displacement air conditioning system compressor, and more particularly, to a control method which protects against compressor damage due to a low refrigerant charge condition.
• Variable displacement refrigerant compressors have been employed in engine driven automotive air conditioning systems in order to reduce engine load variations associated with compressor cycling.
• the compressor displacement is controlled by regulating the compressor crankcase pressure
• a pneumatic control valve integral to the compressor variably connects the compressor crankcase to the inlet (suction) and outlet (discharge) chambers of the compressor.
• the control valve is mechanized with a solenoid valve positioned to achieve the ratiometric control.
• the valve may be linearly positioned by controlling the solenoid current, or pulse-width-modulated at a variable duty cycle to alternately connect the crankcase to the inlet and outlet chambers.
• refrigerant charge As with fixed displacement compressors, internal lubrication is provided by a small amount of oil suspended in the refrigerant
• the amount of refrigerant in the system referred to herein as the refrigerant charge, therefore determines the degree of compressor lubrication as well as the cooling performance of the system. If a significant portion of the refrigerant escapes, compressor lubrication may be insufficient and continued operation under such conditions may severely damage the compressor.
• the present invention is directed to an improved low charge protection method for an electronically controlled variable displacement compressor.
• a low charge test sequence is carried out to monitor the system performance once the system control pressure has been reduced below a specified level.
• the compressor is down-stroked to near-minimum displacement for a predetermined time (such as 20 seconds) or until the system control pressure rises above a reference level. At such point, the compressor is up-stroked to near-maximum displacement to initiate a pull-down phase of the test.
• a failed test is indicated and the count in a nonvolatile counter is incremented, If the pull-down duration exceeds the reference interval, a passed test is indicated, and the count, if any, is decremented.
• the nonvolatile count exceeds a specified threshold, the compressor is disabled and further operation is prevented until the count is reset by a service technician.
• FIG. 1 is a block diagram of an automotive air conditioning system in accordance with the present invention, including a computer-based electronic control unit.
• FIG. 2 is a graph depicting the evaporator pressure in a low charge test according to this invention.
• FIGS. 3A, 3B and 3C are flow diagrams representative of computer program instructions executed by computer-based control unit of FIG. 1 in carrying out the control of this invention.
• the reference numeral 10 generally designates an automotive air conditioning system including a variable displacement refrigerant compressor 12, a condenser core 14, an expansion orifice 16, an evaporator core 18 and an accumulator 20.
• the compressor 12 is driven by the vehicle engine 22 via a belt and pulley drive arrangement generally designated by the reference numeral 24.
• the compressor 12 includes a pulse-width-modulated (PWM) solenoid valve 26 for alternately connecting the crankcase of compressor 12 to the inlet (suction) and outlet (discharge) pressures of the compressor at a controllable duty cycle. This effects a ratiometric control of the crankcase pressure between the inlet and outlet pressures, which in turn, controls the displacement of the compressor 12.
• An electro-magnetic clutch 28 is controlled to selectively engage and disengage the pulley drive arrangement 24.
• An electronic control unit 30 controls the operation of the solenoid valve 26 and clutch 28 via lines 32 and 34, as explained below.
• the PWM duty cycle applied to solenoid valve 26 is inversely related to the resultant change in compressor displacement. That is, relatively high duty cycle energization of the solenoid valve 26 serves to decrease the capacity of, or destroke, the compressor 12, while relatively low duty cycle energization serves to increase the capacity of the compressor 12.
• An intermediate duty cycle energization in the range of approximately 50%-70% maintains the current capacity
• warm pressurized gaseous refrigerant discharged from the engine driven compressor 12 is cooled and liquefied by the condenser 14, which is typically air cooled.
• the orifice 16 rapidly decreases in the pressure of the condensed refrigerant, effecting further cooling of the same prior to its entry into the evaporator 18.
• the refrigerant supplied to the inlet of evaporator 18 is predominantly liquid. Warm air flowing across the evaporator 18 vaporizes or boils the cooled refrigerant therein, thereby cooling the passenger compartment.
• the warmed refrigerant is then discharged to the accumulator 20, which separates out the gaseous portion for return to the inlet of compressor 12.
• the control unit 30 is powered by the vehicle storage battery 36, and generates control signals for the compressor 12 and clutch 28 on lines 32 and 34 in response to various input signals received on lines 40-44.
• the MODE signal on line 40 is obtained from an operator manipulated control head 48, by which the operator designates the desired operating mode: normal (N) or economy (E).
• the control head 48 also serves to position a mix door 50 for regulating the temperature of the conditioned air supplied to the passenger compartment.
• the pressure signal P e on line 42 is generated by a pressure transducer 52 mounted at the outlet of evaporator 18 to sense the pressure of the gaseous refrigerant therein.
• the speed signal N e on line 44 is generated by a speed sensor 45 responsive to the rotary speed of the output shaft 46 of engine 22.
• control unit 30 uses the MODE and N e signals on lines 40 and 44 to develop a control setting, designated herein as a pressure command PCMD for the outlet of the evaporator 18.
• the pressure signal P e on line 42 is used as a feedback parameter, and the control unit 30 energizes the solenoid valve 26 via line 32 at a duty cycle chosen to bring the measured pressure signal P e into correspondence with the pressure command PCMD.
• the compressor displacement is controlled as required to maintain the evaporator outlet pressure Pe at the commanded value PCMD.
• control unit 30 also carries out a test sequence for determining if the refrigerant charge is adequate to protect the compressor 12.
• the test sequence is outlined above and described in detail below in reference to FIGS. 2 and 3A-3B.
• control unit 30 comprises a microcomputer (uC) 54 with both volatile and nonvolatile memory, an Input/Output (I/O) device 56, a pulse-width-modulation (PWM) driver 58, an address and control bus 60 and a data bus 62.
• the I/O device 56 receives the inputs on lines 40-44, and under the control of microcomputer 54, supplies a duty cycle command to the PWM driver 58.
• Flow diagrams representative of the program instructions executed by the microcomputer 54 in carrying out the compressor control and the test sequence of this invention are described below in reference to FIGS. 3A-3C.
• FIG. 2 A typical period of operation according to the present invention is graphically illustrated in FIG. 2, where the evaporator outlet pressure P e is plotted as a function of time. Compressor operation is initiated by energizing clutch 28 at time t 0 , with P e at an initial relatively high value P init .
• the pressure error, PCMD - P e is large, and the control unit 30 up-strokes the compressor 12 to maximize the air conditioning performance.
• the evaporator pressure Pe reaches the command value PCMD at time t 1 , the set-up phase of the low charge test sequence is initiated.
• the control unit 30 initiates the set-up phase of the test sequence, down-stroking compressor 12 to near-minimum displacement. This permits the evaporator outlet pressure P e to increase, as indicated in the interval t 1 -t 2 .
• the rate of increase depends on the ambient temperature and humidity, the evaporator load (fan speed), compressor speed and the refrigerant charge. Under high ambient or high load conditions with normal refrigerant charge, the pressure rises quickly and may reach the entry pressure P LCH in as little as 6.0 seconds. Under low ambient, low load conditions with normal refrigerant charge, however, the pressure may never reach the entry pressure P LCH . For these conditions, the control unit 30 initiates the next (pull-down) phase of the test after a down-stroke time-out of 20 seconds.
• the control unit 30 initiates the pull-down phase of the test sequence, up-stroking the compressor 12 to near-maximum displacement. This produces a decrease in the evaporator outlet pressure P e , as indicated in the interval t 2 -t 4 .
• the pull-down phase is terminated at time t 4 when the compressor 12 has reduced P e by a specified differential, such as 20 PSI. If P e reaches the entry pressure P LCH within the 20-second time-out, as shown in FIG. 2, the pull-down is terminated when P e reaches an exit threshold P LCL , 20 PSI lower than P LCH .
• the pull-down is terminated after an evaporator pressure reduction of 20 PSI without regard to the predefined entry and exit pressures P LCH and P LCL .
• the terms P LCH and P LCL are redefined under such conditions so that the evaporator pressure achieved at the termination of the time-out interval, t 2 , becomes the entry pressure, as described below in reference to FIG. 3C. In either event, the timed interval is initiated when the evaporator pressure P e falls below the entry pressure P LCH .
• the timed interval of the pull-down phase exceeds a reference interval such as 6 seconds, the refrigerant charge is deemed adequate and the test is terminated. However, if the 20 PSI pressure differential is achieved in 6 seconds or less, an inadequate level of refrigerant charge is indicated.
• the relatively fast pull-down occurs when the charge is so low that the refrigerant supplied to the inlet of evaporator 18 is predominantly gaseous. Since there is little or no liquid refrigerant to evaporate, the 20 PSI pressure differential is quickly achieved
• the count in a nonvolatile (electrically erasable or E 2 ) memory location of micro-computer 54 is incremented. If the pull-down duration exceeds the 6-second reference interval, the nonvolatile count, if any, is decremented. When the count exceeds a specified threshold, the compressor 12 is disabled, and further operation is prevented until the nonvolatile count is reset by a service technician when the system is re-charged with refrigerant.
• FIGS. 3A-3C represent computer program instructions executed by the micro-computer 54 of control unit 30 in carrying out the low charge protection method of this invention.
• FIG. 3A depicts a main or executive program loop
• FIGS. 3B-3C together depict a program routine for carrying out the low charge routine.
• the reference numeral 100 generally designates a set of instructions executed at the initiation of each period of vehicle operation for initializing the various memory registers, flags and timer values employed in the control. If the count in the E 2 memory is greater than a threshold count (such as 6), as determined by the decision block 102, the blocks 104 and 106 are executed to de-energize the compressor clutch 28 and set a LOW CHARGE (LC) FAIL flag. In such case, further compressor control is suspended until the E 2 memory location is reset by a service technician.
• a threshold count such as 6
• LC LOW CHARGE
• the refrigerant charge is presumed to be adequate, and the block 108 is executed to reset the low charge test flags LCF, LCFa and LCFb. Thereafter, the blocks 110-116 are sequentially and repeatedly executed, as indicated by the flow diagram lines.
• the system input signals such as Ne, P e and MODE are read at block 110; the pressure command PCMD is determined at block 112; the low charge test logic of this invention is executed at block 114; and the normal compressor displacement control is executed at block 116.
• a detailed description of a representative pressure command determination is given in U.S. Ser. No. 399,039, filed Aug. 28, 1989, now U.S. Pat. No.
• the three flags referred to above are employed to designate the current state of the test.
• Each of the flags is initialized (reset) by the block 108 of FIG. 3A.
• the first flag LCF is set at the initiation of the set-up phase.
• the second flag LCFa is set at the initiation of the pull-down phase.
• the third flag LCFb is set at the termination or completion of the test.
• the decision blocks 120-122 are first executed to determine the status of the low charge test. If the LCFb flag is set, the test has been completed, and the remainder of the routine is skipped. If the LCF flag is not set, the test has not yet started, and the blocks 124-126 are executed to (1) set the LCF flag, (2) down-stroke the compressor 12, and (3) reset the time-out timer as soon as the evaporator outlet pressure P e falls below the pressure command value PCMD, thereby initiating the set-up phase of the test.
• the decision block 128 is executed to determine if the compressor speed (CRPM) is in the range of 1500-4500 RPM. If not, the low charge test cannot be reliably performed, and the block 130 is executed to reset the LCF and LCFa flags, terminating the test.
• CRPM compressor speed
• decision block 134 is executed to determine if the evaporator pressure P e has reached the entry pressure P LCH . If not, the block 136 is executed to increment the time-out timer. If the time-out timer is incremented to a count representing more than approximately 20 seconds before P e reaches the entry pressure P LCH , as determined by the decision blocks 134 and 138, the block 140 is executed to reset the entry pressure P LCH to the current value of P e .
• the exit pressure P LCL is then defined as (P LCH --20 PSI) by block 142, and the block 144 is executed to initiate the pull-down phase of the test.
• block 144 sets the LCFa flag, initiates up-stroking of compressor 12 and resets the timer so that it can be used to time the pull-down interval.
• the decision block 148 is executed to determine if P e has reached the exit pressure P LCL . If not, the block 150 is executed to increment the pull-down timer. If the timer count reaches a value representative of approximately 6 seconds before P e reaches the exit pressure P LCL , as determined by blocks 148 and 152, the refrigerant charge is deemed adequate and the blocks 154-156 are executed to set the LCFb flag and decrement the E 2 count, if any, completing the routine.
• the refrigerant charge is deemed inadequate to protect the compressor 12, and the block 158 is executed to increment the E 2 count. Until the incrementing causes the count to exceed a reference count such as 6, as determined at block 160, continued compressor operation is permitted and the block 162 is executed to set the LCFb flag. When the count reaches the reference count, the block 164 is executed to set the LOW CHARGE (LC) FAIL flag and to deenergize the clutch 28. Further compressor operation is suspended until the E 2 memory location is reset by a service technician.
• LC LOW CHARGE
• the low charge protection of this invention provides a reliable indication of the adequacy of the refrigerant charge, and protects the variable displacement compressor 12 from damage due to extended operation at low refrigerant charge levels.
• the system may pass and fail successive low charge tests, and the E 2 count effectively integrates the low charge indications over time. In this way, the routine of this invention provides adequate protection of the compressor without causing unnecessary or nuisance interruptions.

Landscapes

• Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Mechanical Engineering (AREA)
• Thermal Sciences (AREA)
• General Engineering & Computer Science (AREA)
• Air-Conditioning For Vehicles (AREA)

Abstract

Description

Claims (6)

Priority Applications (1)

Applications Claiming Priority (1)

Publications (1)

Family

ID=24244797

Family Applications (1)

Country Status (1)

Cited By (58)

Citations (6)

• 1990
  • 1990-08-02 US US07/562,100 patent/US5009074A/en not_active Expired - Fee Related

Patent Citations (6)

Similar Documents

Legal Events